Method and system to segment depth images and to detect shapes in three-dimensionally acquired data

ABSTRACT

A method and system analyzes data acquired by image systems to more rapidly identify objects of interest in the data. In one embodiment, z-depth data are segmented such that neighboring image pixels having similar z-depths are given a common label. Blobs, or groups of pixels with a same label, may be defined to correspond to different objects. Blobs preferably are modeled as primitives to more rapidly identify objects in the acquired image. In some embodiments, a modified connected component analysis is carried out where image pixels are pre-grouped into regions of different depth values preferably using a depth value histogram. The histogram is divided into regions and image cluster centers are determined. A depth group value image containing blobs is obtained, with each pixel being assigned to one of the depth groups.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 11/349,311, filed on Feb. 6, 2006, which claims thebenefit of U.S. provisional patent application No. 60/651,094, filedFeb. 8, 2005, both of which are incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates generally to recognizing objects acquired inthree-dimensionally acquired data, including data acquired from imagesensors, for example depth or range finders, image mapping sensors,three-dimensional image capture sensors including capture of images withcolor perception not limited by human color perception.

Electronic camera and range sensor systems that provide a measure ofdistance from the system to a target object are known in the art. Manysuch systems approximate the range to the target object based uponluminosity or brightness information obtained from the target object.Some such systems are passive and respond to ambient light reflectedfrom the target object, while other systems emit and then detect emittedlight reflected from the target object. However luminosity-based systemsmay erroneously yield the same measurement information for a distanttarget object that happens to have a shiny surface and is thus highlyreflective, as for a target object that is closer to the system but hasa dull surface that is less reflective.

A more accurate distance measuring system is a so-called time-of-flight(TOF) system. FIG. 1 depicts an exemplary TOF system, as described inU.S. Pat. No. 6,323,942 entitled CMOS-Compatible Three-Dimensional ImageSensor IC (2001), which patent is incorporated herein by reference asfurther background material. TOF system 100 can be implemented on asingle IC 110, without moving parts and with relatively few off-chipcomponents. System 100 includes a two-dimensional array 130 of pixeldetectors 140, each of which has dedicated circuitry 150 for processingdetection charge output by the associated detector. In a typicalapplication, array 130 might include 100×100 pixels 230, and thusinclude 100×100 processing circuits 150. IC 110 also includes amicroprocessor or microcontroller unit 160, memory 170 (which preferablyincludes random access memory or RAM and read-only memory or ROM), ahigh speed distributable clock 180, and various computing andinput/output (I/O) circuitry 190. Among other functions, controller unit160 may perform distance to object and object velocity calculations.

Under control of microprocessor 160, a source of optical energy 120 isperiodically energized and emits optical energy via lens 125 toward anobject target 20. Typically the optical energy is light, for exampleemitted by a laser diode or LED device 120. Some of the emitted opticalenergy will be reflected off the surface of target object 20, and willpass through an aperture field stop and lens, collectively 135, and willfall upon two-dimensional array 130 of pixel detectors 140 where animage is formed. Each imaging pixel detector 140 measures both intensityor amplitude of the optical energy received, and the phase-shift of theoptical energy as it travels from emitter 120, through distance Z totarget object 20, and then distance again back to imaging sensor array130. For each pulse of optical energy transmitted by emitter 120, athree-dimensional image of the visible portion of target object 20 isacquired.

Emitted optical energy traversing to more distant surface regions oftarget object 20 before being reflected back toward system 100 willdefine a longer time-of-flight than radiation falling upon and beingreflected from a nearer surface portion of the target object (or acloser target object). For example the time-of-flight for optical energyto traverse the roundtrip path noted at t1 is given by t1=2·Z1/C, whereC is velocity of light. A TOF sensor system can acquirethree-dimensional images of a target object In real time. Such systemsadvantageously can simultaneously acquire both luminosity data (e.g.,signal amplitude) and true TOF distance measurements of a target objector scene.

As described in U.S. Pat. No. 6,323,942, in one embodiment of system 100each pixel detector 140 has an associated high speed counter thataccumulates clock pulses in a number directly proportional to TOF for asystem-emitted pulse to reflect from an object point and be detected bya pixel detector focused upon that point. The TOF data provides a directdigital measure of distance from the particular pixel to a point on theobject reflecting the emitted pulse of optical energy. In a secondembodiment, in lieu of high speed clock circuits, each pixel detector140 is provided with a charge accumulator and an electronic shutter. Theshutters are opened when a pulse of optical energy is emitted, andclosed thereafter such that each pixel detector accumulates charge as afunction of return photon energy falling upon the associated pixeldetector. The amount of accumulated charge provides a direct measure ofround-trip TOF. In either embodiment, TOF data permits reconstruction ofthe three-dimensional topography of the light-reflecting surface of theobject being imaged.

Many factors, including ambient light, can affect reliability of dataacquired by TOF systems. As a result, the transmitted optical energy maybe emitted multiple times using different systems settings to increasereliability of the acquired TOF measurements. For example, the initialphase of the emitted optical energy might be varied to cope with variousambient and reflectivity conditions. The amplitude of the emitted energymight be varied to increase system dynamic range. The exposure durationof the emitted optical energy may be varied to increase dynamic range ofthe system. Further, frequency of the emitted optical energy may bevaried to improve the unambiguous range of the system measurements.

U.S. Pat. No. 6,580,496 entitled Systems for CMOS-CompatibleThree-Dimensional Image-Sensing Using Quantum Efficiency Modulation(2003) discloses a sophisticated system in which relative phase (φ)shift between the transmitted light signals and signals reflected fromthe target object is examined to acquire distance z. Detection of thereflected light signals over multiple locations in a pixel array resultsin measurement signals that are referred to as depth images. FIG. 2Adepicts a system 100′ according to the '496 patent, in which anoscillator 115 is controllable by microprocessor 160 to emit highfrequency (perhaps 200 MHz) component periodic signals, ideallyrepresentable as A·cos(ωt). Emitter 120 transmitted optical energyhaving low average and peak power in the tens of mW range, which emittedsignals permitted use of inexpensive light sources and simpler, narrowerbandwidth (e.g., a few hundred KHz) pixel detectors 140′. Unlessotherwise noted, elements in FIG. 2A with like reference numerals toelements in FIG. 1 may be similar or identical elements.

In system 100′ there will be a phase shift φ due to the time-of-flight(TOF) required for energy transmitted by emitter 120 (S₁=cos(ωt)) totraverse distance z to target object 20, and the return energy detectedby a photo detector 140′ in array 130′, S₂=A·cos(ωt+φ), where Arepresents brightness of the detected reflected signal and may bemeasured separately using the same return signal that is received by thepixel detector. FIGS. 2B and 2C depict the relationship between phaseshift φ and time-of-flight, again assuming for ease of description asinusoidal waveform.

The phase shift φ due to time-of-flight is:φ=2·ω·z/C=2·(2πf)·z/C

where C is the speed of light 300,000 Km/sec. Thus, distance z fromenergy emitter (and from detector array) to the target object is givenby:z=φ·C/2ω=φ·C/{2·(2πf)}

As noted above, many types of three-dimensional imaging systems areknown in the art. But even if reasonably accurate depth images can beacquired by such systems. Further, it can be important to rapidlyanalyze the acquired data to discern whether objects are present thatmay require immediate response. For example, systems such as describedin the '496 patent may be used as robotic sensors to determine whethercertain objects are nearby whose presence may dictate the immediateshut-down of equipment for safety reasons. Systems including systemsdescribed in the '496 patent may be used within motor vehicle to helpthe vehicle operator quickly recognize objects whose presence mayrequire immediate response, e.g., braking to avoid hitting pedestriansin the vehicle's path.

What is needed is a method and system useable with existing imageacquisition systems to more rapidly and more reliably identify objectswithin the acquired data whose presence may dictate certain responses.The present invention provides such methods and systems.

SUMMARY OF THE INVENTION

The present invention is usable with systems that acquire depth images,and provides methods and systems to analyze such images. The presentinvention segments the images to detect shapes or so-called blobstherein to help rapidly identify objects in the acquired image. Thepresent invention can be practiced on depth images, without regard towhether they were acquired with so-called stereographic cameras, laserrange sensors, time-of-flight sensors, or with more sophisticatedimaging systems, such as time-of-flight systems exemplified by U.S. Pat.No. 6,323,942, or phase-shift systems exemplified by U.S. Pat. No.6,580,496.

In one aspect, the acquired depth or range image is segmented intogroups of objects that are logically connected within the image. Forexample intensity-based pixels, perhaps acquired with a conventionalcamera, may be labeled according to color. More preferably, pixelsacquired from a true z-depth measuring system are labeled such thatlogically connected pixels are assigned the same depth or z-value.Logical connectivity can relate to various characteristics of theacquired image. For example, with an intensity-based image such asacquired by a conventional camera, pixels can be labeled according tocolor. An image of a human wearing black pants and a red shirt could beseparated into two sub-images. However a problem common withintensity-based images is that if there is occlusion or overlap betweenobjects in the image, the grouping or segmentation may be unsuccessfulas there is no true depth perception.

As applied to true z-depth data images, segmenting according to anembodiment of the present invention is such that neighboring pixels inthe image that have similar depths are given a common label. As usedherein, “blobs” may be constructed from the labeled image, where a“blob” is a group of pixels having the same label. Preferably each blobwill correspond to a different object, and blobs can be modeled asprimitives of different shapes (e.g., a circle, a rectangle, etc.), oras pre-defined objects, e.g., a motor vehicle, a human, an animal.

Using embodiments of a modified connected component analysis, thepresent invention can recognize the presence of blobs within an image,to more rapidly correctly characterize the image. In some embodiments,image pixels are pre-grouped into regions of different depth values,preferably using a depth value histogram, which is itself divided intoregions. Image cluster centers can then be determined and a depth groupvalue image obtained, in which each pixel is assigned to one of thedepth groups. Such modified connected component analysis is then carriedout to identify blobs or objects within the image data. Blob classes maybe defined for the application at hand, to help rapidly identify objectsin the acquired image. For example, when used with a system in a motorvehicle to identify potential driving hazards, one class of blobs may becharacterize pedestrians, other vehicles, and the like.

Other features and advantages of the invention will appear from thefollowing description in which the preferred embodiments have been setforth in detail, in conjunction with their accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a time-of-flight range finding system,according to the prior art;

FIG. 2A depicts a phase-shift intensity and range finding system,according to the prior art;

FIG. 2B depicts a transmitted periodic signal with high frequencycomponents transmitted by the system of FIG. 2A, according to the priorart;

FIG. 2C depicts the return waveform with phase-delay for the transmittedsignal of FIG. 2B, according to the prior art;

FIG. 3 depicts a generic image-acquisition system provided with asegmenter-image recognizer system, according to an embodiment of thepresent invention;

FIG. 4A depicts a depth image comprising three non-adjacent objectregions of different depths, showing exemplary segmentation with objectsbearing label 1, label 2, and label 3, according to an embodiment of thepresent invention;

FIG. 4B depicts a depth image comprising object regions of differentdepths that are adjacent to each other, with the left image portiondepicting a magnified view of the transition or overlapping objectregions, according to an embodiment of the present invention;

FIG. 4C depicts the depth image of FIG. 4B, after application of amodified connected component analysis algorithm, according toembodiments of the present invention;

FIG. 5 depicts exemplary pseudocode implementing a two-way neighborhoodgrouping by which connected black or white pixels are assigned a sameblack or white label, according to an embodiment of the presentinvention;

FIG. 6 depicts exemplary pseudocode implementing a modified connectedcomponent analysis by which depth values of pixels are implemented andneighboring pixels with similar depth values are assigned a same label,according to an embodiment of the present invention;

FIG. 7 depicts exemplary pseudocode implementing a method in which wherepixels are pre-grouped into regions of different depth values using ahistogram of depth values, according to an embodiment of the presentinvention; and

FIG. 8 is an exemplary histogram of the figure of FIG. 4B, using theprocedure depicted in FIG. 7, according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 depicts a three-dimensional imaging system 200′ comprising adepth image data acquisition system 210, and a segmenter-imagerecognizer system 220, according to the present invention. Depth imagedata acquisition system 210 may be almost any system that acquiresimages with three-dimensional depth data. As such system 210 couldinclude time-of-flight systems such as shown in FIG. 1, phase-shiftdetection systems such as shown in FIG. 2A, laser range sensors, andstereographic camera systems, among other. As such, energy emanatingfrom system 200 is drawn with phantom lines to denote that system 200may not actively emit optical energy but instead rely upon passiveoptical energy from ambient light.

System 210 may be implemented in many ways and can, more or less,provide a stream of output information (DATA) that includes a measure ofdistance z to a target object 20. Such DATA may include information asto target objects that might not be readily identifiable from the rawinformation. However, according to the present invention,segmenter-image recognizer system 220 can process the information streamusing an algorithmic procedure 240 stored in memory 230, and executableby a microprocessor 250. If depth image data acquisition system 210 isimplemented according to the systems of FIG. 1 or FIG. 2A, functions ofsome or all elements of segmenter-image recognizer system 220 may beprovided elsewhere. For example, processing tasks of microprocessor 250in FIG. 3 may in fact be carried out by microprocessor 160 in FIG. 1 orFIG. 2A, and/or storage facilities provided by memory 230 in FIG. 3 maybe carried out by memory 170 in FIG. 1 or FIG. 2A.

Referring to FIG. 4A, assume that data processed by generic system 200in FIG. 3A produced image 300, an image comprising three objects: aperson 310, a vehicle 320, and background 330. When software 240implementing an algorithm according to the present invention isexecuted, e.g., by microprocessor 250, segmentation and identificationof the data acquired by system 200 is carried out, according to anembodiment of the present invention.

Software 240 carries out segmentation by labeling pixels comprisingimage 300 such that connected pixels are assigned the same value. Image300 in FIG. 4A may be grouped using three labels: person 310 is assignedlabel 1, vehicle 320 is assigned label 2, and background 330 is assignedlabel 3. In one embodiment of the present invention, if image 300 is anintensity image, perhaps acquired from a stereographic camera system210, pixels comprising the image may be grouped according to color. Forexample if person 310 is wearing a shirt of one color and pants ofanother, the image of his body could be separated into an upper segment(the shirt) and a lower segment (the pants), with separate labelsassigned to each of these two segments. Note that if occlusions arepresent in the image, e.g., overlapping objects, segment grouping mightbe unsuccessful as there is no perception of depth.

Assume now that image 300 in FIG. 4A has been acquired with a depthimage data acquisition system 210 that can acquire true z-depth data, asystem such as shown in FIG. 1 or FIG. 3A, for example. As such, in suchacquisition systems, the depth image provides the z-depth value for eachpixel comprising the image. Another embodiment of the present inventiongroups (or segments) portions of the image from such acquisition systemssuch that neighboring pixels that have similar z-depths are given thesame labels. Groups of pixels bearing the same label and that areconnected are termed “blobs”, where each such blob corresponds to adifferent object. If image 300 in FIG. 4A is now considered to be adepth image, then it is evident that person 310 is at a differentz-distance from system 200 than is vehicle 320 or background 330.

According to embodiments of the present invention, once blobs aredefined, they may be modeled or quantized into variously shapedprimitives, for example, a circle, a rectangle, or as predefinedobjects, for example, a person, an animal, a vehicle.

Consider further depth image 300 in FIG. 4A with respect to variousembodiments of the present invention. As used herein, a connectedcomponent analysis is an imaging method or algorithm by which certainproperties and proximities of a pixel are used to group pixels together.For instance, suppose image 300 comprises black and white image pixels.An algorithm or method according to the present invention preferablygroups the pixels by labeling connected white or connected black pixelswith the same label. A description of such a method as applied tofour-neighbor pixel connectivity (e.g., left, right, up, down) will nowbe given with reference to the exemplary pseudocode depicted in FIG. 5.

In the embodiment exemplified by the pseudocode of FIG. 6, a modifiedconnected component analysis is defined in which values of the z-depthpixels are examined. According to this embodiment, pixels are given thesame value as neighboring pixels with similar depth values. Without anylimitation, the algorithm pseudocode shown in FIG. 6 exemplifiesfour-connectivity, although of course eight-connectivity or othermeasure of connectivity could instead be used. By four-connectivity, itis meant that with respect to a given pixel in array 130 or 130′, thealgorithm code will inspect the pixel above in the previous row (if any)and the pixel below in the next row (if any), in defining the potentialneighborhood of interest. In the boundary conditions, a defaultassumption for the property of the missing pixels is made.

As shown in FIG. 6, z-depth values for a given pixels (r,c forrow,column) in the array is tested to see whether its measured z-depthvalue is within a threshold depth value compared to its left neighbor.If not less than the threshold, the comparison is repeated with its topneighbor. Pixels whose z-depth values are less than the threshold aretentatively labeled as having the same label value. If the tested pixelsstill exceed the threshold, a new label is generated and assigned topixel (r,c). Then z-values of the top and the left pixels are compared.If the z-values are within the threshold, those pixels are labeled asbeing connected. The test is repeated until pixels in the array arelabeled and their connectivities are established. The threshold depthvalue can be obtained by heuristic methods depending on the type ofapplication.

The method exemplified in the embodiment of FIG. 6 thus partitions theimage into connected regions having similar depth properties. Inpractice, some depth sensors might produce transition pixels when tworegions in the depth image are adjacent to each other but have differentz-depth values. FIG. 4B depicts such juxtaposition, with the left-handportion of the figure providing a zoomed-in depiction of the overlappingtransition pixels. As indicated by the overlapping cross-hatching in thezoomed-in region of FIG. 4B, embodiments of the present invention assignsome of the transition pixels depth values in between the depth valuesassociated with the two regions in question. The embodiment of thepresent invention exemplified by FIG. 6 might combine the two regionsthrough the transition pixels, for example by assigning a common labelto the entire superimposed image, as shown in FIG. 4C. Thus in FIG. 4C asingle object 2 is defined, comprising the human 310 and the motorvehicle 320.

FIG. 7 depicts exemplary pseudocode used in an embodiment of the presentinvention that takes a different approach to overlapping image objects.More specifically, the embodiment represented by the exemplarypseudocode in FIG. 7 implements a modified connected component analysisthat uses a derived property for each pixel as belonging to a number ofdepth groups.

Such grouping is carried out by software 240 obtaining a histogram ofthe z-depth values. The histogram may be stored in memory 230, andpreferably is divided into regions using a k-means algorithm, whichalgorithms are known in the art. For instance, the image of FIG. 4Bwould have a histogram represented by FIG. 8, which has twodistinguishable regions: a left peak representing person 310 (label 1),and a right peak representing vehicle 320 (label 2).

More specifically, according to one embodiment of the present invention,the histogram is grouped into two regions, whereafter image clustercenters are determined using a known procedure, for example, a k-meansalgorithm as noted above. As indicated by the exemplary pseudocode ofFIG. 7, a depthGroupValue image may now be obtained, with each pixelbeing assigned to one of the depth groups. In FIG. 7, the algorithm ofFIG. 6 is modified to use depth group value of a pixel (r,c) instead ofits original depth value. FIG. 7 depicts an embodiment of the presentinvention wherein software 240 (preferably stored in memory 230) andexecuted by a microprocessor, e.g., microprocessor 250 outputs data inwhich pixels are pre-grouped into regions of different depth values.

Thus application of the modified connected component algorithm depictedin FIG. 7 can result in an image as shown in FIG. 4B, withoutconsidering the zoomed-portion of FIG. 4B.

Applicants's pre-grouping algorithm can encounter difficulty when theimage under consideration is a complicated scene having many z-depthregions. In such cases, it is preferable to work on sub-divided regionsof the image. For example, in one embodiment of the present invention,pre-grouping of the image can be accomplished by first dividing theimage into regions. Without limitation, the image may be divided intoseveral sub-regions, which some or all sub-regions may overlap othersub-regions. In one embodiment, the sub-regions are defined to berectangles that are equally divided in the image. In this embodiment,pre-grouping is applied within each rectangle, for example (and withoutlimitation) using a k-means algorithm. Next, applicants's connectedcomponent analysis is carried out on each sub-region.

The output of the above-described segmentation procedure is a labeledimage, for example image 300 shown in FIG. 4B. “Blobs” may beconstructed from the labeled image, a blob being a group of pixels withthe same label. The exemplary algorithm procedures described hereinpreferably are used to locate these blobs within an image. In practice,each blob preferably is modeled according to the applicationrequirements. By way of example and without limitation, blobs can bemodeled as rectangles, where pixel start row, start column, end row, andend column are defined by the boundary of the blob. For instance,different colors can be used to display rectangles commensurate with theaverage depth value of the blob. Perhaps a red color could be used foran object that is near, and a green color used for an object that isfar. By way of further example, a human might be modeled as rectanglesof specific height and weight. Embodiments of the present invention canthen advantageously track objects by noting blob movement from image toimage. Such image tracking advantageously yields a result more quicklythan if pixel-by-pixel tracking were used, image to image.Understandably, blob tracking can substantially reduce the computationalrequirements to implement a tracking algorithm according to the presentinvention.

It is understood that memory 230 may store a variety of pre-definedblobs, appropriate to the intended use of overall system 200. Forexample, system 200 may be disposed in a motor vehicle as part of awarning system, to reduce the likelihood of a collision between themotor vehicle and objects. In such application, predefined models may bestored in memory representing large objects such as another motorvehicle, smaller objects such as a human or animal, column-shapedobjects such as a sign post or a traffic light pillar. The ability ofthe present invention to rapidly recognize such objects from depth andimage data acquired by system 210 can enable data from system 200 toalert the operator of the motor vehicle containing the system, asindicated in FIG. 3. If overall system 200 recognizes what the presentinvention determines to be a human in the path of the motor vehiclecontaining the system, the data output signal can be used toautomatically sound the vehicle's horn, flash headlights, or even toapply the vehicle's brakes.

In other applications, overall system 200 may image occupants of avehicle containing the system, for example to determine the size of anoccupant in the front passenger seat of the vehicle. In suchapplication, memory 230 might store predefined models of objects likelyto be in the passenger seat, for example, a large adult, a small adult,a child, a baby, a dog, a package. In an emergency situation, overallsystem 200 may be used to intelligently deploy an air bag such that theair bag can deploy normally if the object in the front passenger seat isdetermined from blob analysis to be an adult of suitable size. On theother hand, if overall system 200 determines that from blob analysisthat the object in the passenger seat is an infant, the data signal fromthe overall system may command the air bag not to deploy, wherenon-deployment is considered the safer alternative for an infant.

The above-described applications must be understood to be merelyexemplary. The present invention would also have utility in an intrusiondetection system, a factory robotic control system, and so forth.

Modifications and variations may be made to the disclosed embodimentswithout departing from the subject and spirit of the invention asdefined by the following claims.

What is claimed is:
 1. A method for use in tracking movement of at leastone object in a series of images, comprising: (a) receiving multipleimages including image depth data from an array of pixels acquired bythree-dimensional imaging system, each image including z-depth data thatcomprises a z-depth value of each pixel from at least a fraction ofoptical energy emitted by the imaging system reflected by at least oneobject, said image depth data generated by comparing emitted opticalenergy and the fraction of emitted optical energy detected by saidsystem, and for each of the multiple images: (a1) examining at leastsome of said image depth data captured during a single data acquisition;(a2) using a depth value histogram that records the z-depth value ofeach pixel of one or more regions, forming segments by grouping togetherthose of said pixels whose acquired said data captured at step (a1) haveat least one common characteristic selected from a group consisting ofdepth distance, depth distance change, object size, and object shape,and assigning pixels in the group a single image cluster center having adepth group value, where the single image cluster center is thegeometric center of said group of pixels; (a3) assigning on aper-segment basis a label to pixels comprising each of said segmentsformed in step (a2), the pixels so labeled enabling recognition of saidat least one object with image depth data; and (b) tracking motion ofthe at least one object relative to the system.
 2. The method of claim 1wherein said receiving includes acquiring, in said array of pixels,intensity data; and step (a2) includes forming segments by groupingtogether pixels based upon their acquired intensity data.
 3. The methodof claim 1 wherein for each of the multiple images, the method furtherincludes steps: (a4) providing at least one pre-defined blob; (a5)examining said segments formed in step (a2) to determine presence insaid image data of a blob matching a pre-defined blob provided at step(a4); and (a6) providing for output a signal commensurate with detectionof a recognized pre-defined blob in said image data.
 4. The method ofclaim 1 wherein step (a2) includes forming segments by grouping togetherpixels based upon their acquired z-depth data, wherein pixelsrepresenting an overlapping portion of objects in said image data areassigned a single label.
 5. The method of claim 1 wherein step (a2)includes forming segments by grouping together pixels based upon theiracquired z-depth data, wherein pixels representing an overlappingportion of objects in said image data are assigned intermediate valuesassociated with said objects.
 6. The method of claim 1 wherein said step(b) comprises tracking movement of said at least one object by notingmovement of commonly labeled pixels.
 7. The method of claim 1 whereinthe method includes tracking movement of at least two objects, andwherein the three-dimension imaging system comprises a time-of-flightimaging system.
 8. An image-analyzing system for tracking objects basedon image data, the system including: a three-dimensional imaging systemhaving an emitter of optical energy, an array of pixels receiving atleast a fraction of the emitted optical energy reflected by at least oneobject; a depth image data acquisition system operable to generate saidimage depth data by comparing emitted optical energy and detectedoptical energy by said array of pixels; and a processor and memory, thememory including code operable to instruct the processor to trackmovement of the at least one object by analyzing multiple images, theanalyzing including, for each image: (a) examining at least some of saidimage depth data captured during a single data acquisition; (b) formingsegments by grouping pixels in each image having a depth distance withina threshold distance from each other and assigning pixels in the group asingle image cluster center having a depth group value, where the singleimage cluster center is the geometric center of said group of pixels,and wherein each pixel in the group is assigned the depth group value ofthe image cluster center; (c) assigning on a per-segment basis a labelto pixels comprising each of said segments formed to enable recognitionof said at least one object with a single acquisition of image depthdata; and (d) for the multiple images, tracking movement of said atleast one object by noting movement of commonly labeled pixels.
 9. Theimage-analyzing system of claim 8 wherein the memory is operable tostore at least one pre-defined blob comprising a set of commonly labeledpixels; and code instructing the processor to: compare said segmentsformed for each image with contents of said memory to determine presencein said image data of a blob matching a pre-defined blob stored in saidmemory; and provide an output signal commensurate with detection of arecognized pre-defined blob in said image data.
 10. The image-analyzingsystem of claim 9 further including a capability to track at least twoobjects.
 11. The image-analyzing system of claim 8 wherein saidthree-dimensional imaging system is selected from a group consisting of(i) a time-counting time-of-flight system, and (ii) a time-of-flightsystem employing phase-shift z-depth measurement.
 12. Theimage-analyzing system of claim 9, wherein: said array of pixels isfurther configured to acquire intensity data in a single acquisition ofimage depth data; and said forming segments by grouping is performedbased upon at least one pixel characteristic selected from a groupconsisting of intensity data acquired by said pixels, z-depth dataacquired by said pixels, and z-depth acquired data using pre-determinedz-depth values.
 13. The image-analyzing system of claim 9, wherein saidforming segments by grouping includes grouping together pixels that arewithin a number k of neighboring pixels, where k is selected from agroup consisting of k=4 and k=8.
 14. The image-analyzing system of claim9, wherein said forming segments by grouping includes grouping togetherpixels based upon their acquired z-depth data, wherein pixelsrepresenting an overlapping portion of objects in said image data areassigned a single label.
 15. The image-analyzing system of claim 9,wherein said forming segments by grouping includes grouping togetherpixels based upon their acquired z-depth data, wherein pixelsrepresenting an overlapping portion of objects in said image data areassigned intermediate values associated with said objects.
 16. Theimage-analyzing system of claim 9, wherein said forming segments bygrouping includes grouping together pixels based upon their acquiredz-depth data, and does so in a manner selected from a group consistingof (i) pre-grouping pixels based upon a histogram record of z-depthvalues of pixels in said array, and (ii) pre-grouping pixels based upontheir acquired z-depth data, wherein pixels are pre-grouped based upon ahistogram record of z-depth values of pixels in said array, where saidhistogram is divided into at least two sub-regions and image clustercenters are determined such that each relevant pixel of said pixels insaid array is assigned to a z-depth-determined group.
 17. A computerreadable hardware storage device including instructions causing amicroprocessor to track movement of at least one object in a series ofimages, the instructions comprising: (a) receiving multiple imagesincluding image depth data acquired by three-dimensional imaging system,image depth data comprising a z-depth value of each of a plurality ofpixels, and for each of the multiple images: (a1) examining at leastsome of said image depth data; (a2) using a depth value histogram of oneor more regions, wherein the depth value histogram records the z-depthvalues of pixels in the one or more regions, forming segments bygrouping together those of pixels in the image data whose acquired saiddata captured at step (a) have at least one common characteristicselected from a group consisting of depth distance, object size, andobject shape, and assigning pixels in the group a single image clustercenter having a depth group value, where the single image cluster centeris the geometric center of said group of pixels, and wherein each pixelin the group is assigned the depth group value of the image clustercenter; (a3) assigning on a per-segment basis a label to pixelscomprising each of said segments formed in step (a2), the pixels solabeled enabling recognition of said at least one object with anacquisition of image depth data; and (b) tracking motion of the at leastone object relative to the system over the multiple images.
 18. Thetangible computer readable storage device of claim 17 wherein the stepof tracking is performed by noting movement of commonly labeled pixels.19. The tangible computer readable storage device of claim 18 furtherincluding instructions comprising: storing in a memory at least onepre-defined blob comprising a set of commonly labeled pixels; comparingsaid segments formed for each image with contents of said memory todetermine presence in said image data of a blob matching a pre-definedblob stored in said memory; and providing an output signal commensuratewith detection of a recognized pre-defined blob in said image data. 20.The computer readable storage device of claim 19 further includingtracking at least two objects.